Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element. An inorganic electroluminescent element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element. An organic electroluminescent element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence-phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
However, in an organic electroluminescent element in view of the future practical application, desired has been development of an organic electroluminescent element which efficiently emits at a high luminance with a low electric consumption.
From this reason, many types of organic electroluminescent elements have been disclosed (for example, refer to Patent Document 1). In the case of utilizing emission from an excited singlet as described above, since a generation ratio of a singlet exciton to a triplet exciton is 1/3, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of an external quantum efficiency (ηext) of taking out light is said to be 5%.
However, since an organic electroluminescent element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (for example, refer to Non-Patent Document 1), researches on materials exhibiting phosphorescence at room temperature have come to be active (for example, refer to Patent Document 2).
Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet, which is principally 4 times of the case of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application.
For example, many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied (for example, refer to Non-Patent Document 2).
In each case, the luminance of emitted light and the light emission efficiency are considerably improved compared with usual devices since the emitted light is derived from phosphorescence when the light emission device is prepared by the above materials. There is a problem, however, that the lifetime of light emission is shorter than that of the usual devices. As above-mentioned, it is the present condition that the properties applicable to practical use cannot be sufficiently attained yet in the high efficiency phosphorescent light emission material since the wavelength of emitted light is difficultly shifted to shorter side and the light emission lifetime is difficultly improved.
The properties applicable to practical use cannot be sufficiently attained yet in the high efficiency phosphorescent light emission material since the wavelength of emitted light is difficultly shifted to shorter side and the light emission lifetime is difficultly improved.
Besides, metal complexes having phenylpyrrazole as the ligand are known, (for example, refer to Patent Documents 3 and 4). Though the light emission lifetime is improved by the phenylpyrrazole compound having the substituting form of the phenyl group to phenylpyrrazole disclosed here, the improvement is not sufficient and there is still room for improvement from the view point of the light emission efficiency.
On the other hand, it is known that the vacuum vapor deposition method usually applied to production of organic electroluminescent element using a low molecular weight compound causes problems in aspects of the equipment and energy efficiency on the occasion of making large the area of the organic electroluminescent element. Printing methods including an ink-jet method and a screen printing method and coating methods including a spin coat method and a cast coating method are considered preferable. Known phosphorescent emission materials suitable for printing methods or coating methods such as spin coat method and cast coating method are organic metal complexes having a dendrimer portion (for example, refer to Patent Document 5) and organic metal complexes each fixed in a polymer chain (for example, refer to Patent Document 6). However, there is left room to be improved from the viewpoint of the lifetime and efficiency of the light emission.
Patent Document 1: JP-A 3-255190
Patent Document 2: U.S. Pat. No. 6,097,147
Patent Document 3: WO 04/085450
Patent Document 4: LP-A 2005-53912
Patent Document 5: WO 02/066552
Patent Document 6: JP-A 2003-342235
Non-patent Document 1: M. A. Baldo et al., Nature, 395, pp. 151-154, (1998)
Non-patent Document 2: S. Lamansky et al., J. Am. Chem. Soc. 123, p. 4304